Converter and power conversion device including the same

ABSTRACT

A converter includes a first diode (D 1 ) having its anode and cathode connected to input terminal (T 0 ) and a first output terminal (T 1 ), respectively, a second diode (D 2 ) having its anode and cathode connected to a second output terminal (T 2 ) and an input terminal (T 0 ), respectively, and a bidirectional switch connected between the input terminal (T 0 ) and a third output terminal (T 3 ). The bidirectional switch includes third to sixth diodes (D 3  to D 6 ) and a transistor (Q 1 ). The first diode (D 1 ), the second diode (D 2 ), and the transistor (Q) are each formed of a wide-bandgap semiconductor, and the third to sixth diodes (D 3  to D 6 ) are each formed of a semiconductor other than wide-bandgap semiconductors.

TECHNICAL FIELD

The present invention relates to a converter and a power conversiondevice including the same, and more specifically to a converter thatconverts alternating current (AC) voltage into first to third directcurrent (DC) voltages and a power conversion device including the same.

BACKGROUND ART

Japanese Patent Laying-Open Mo. 2011-78296 (PTD 1) discloses a converterincluding four transistors and six. diodes to convert AC voltage intohigh voltage, low voltage, and intermediate voltage. In this converter,among the six diodes, two diodes performing reverse recovery operationare formed of a wide-bandgap semiconductor, thereby achieving reductionof recovery loss in addition, four diodes that do not perform reverserecovery operation are formed of a semiconductor other than wide-bandgapsemiconductors, thereby achieving lower costs.

CITATION LIST Patent Document

-   PTD 1: Japanese Patent Laying-Open No. 2011 -78296

SUMMARY OF INVENTION Technical Problem

Unfortunately, the conventional converter includes many semiconductorelements and therefore has a larger size and costs much. Moreover, theloss in four transistors is large.

The main object of the present invention is therefore to provide acompact and low-cost converter with small loss and a power conversiondevice including the same.

Solution to Problem

A converter according to the present invention convertsalternating-current voltage applied to an input terminal into first tothird direct-current voltages to be output to first to third outputterminals, respectively. The converter includes a first diode having ananode and a cathode connected to the input terminal and the first outputterminal, respectively, a second diode having an anode and a cathodeconnected to the second output terminal, and the input terminal,respectively, and a first bidirectional switch connected between theinput terminal and the third output terminal. The first direct-currentvoltage is higher than the second direct-current voltage, and the thirddirect-current voltage is an intermediate voltage between the firstdirect-current voltage and the second direct-current voltage. The firstbidirectional switch includes third to sixth diodes and a firsttransistor. The third and fourth diodes have anodes connected to theinput terminal and the third output terminal, respectively, and havecathodes connected together to a first elect rode of the firsttransistor. The fifth and sixth diodes have cathodes connected to theinput terminal and the third output terminal, respectively, and haveanodes connected together to a second electrode of the first transistor.The first transistor is turned on and off in a predetermined cycle. The

first diode, the second diode, and the first transistor are each formedof a wide-bandgap semiconductor. The third to sixth diodes are eachformed of a semiconductor other than wide-bandgap semiconductors.

Advantageous Effects of Invention

The converter according to the present invention is constituted with onetransistor and six diodes. Since the number of semiconductor elements isthus fewer than the conventional one, size reduction and lower costs ofthe device can be achieved. Furthermore, since the first and seconddiodes that perform reverse recovery operation and the first transistorthat switches current a reformed of a wide-bandgap semiconductor,reduction of switching loss and recovery loss can be achieved. Since thethird to sixth diodes that do not perform reverse recovery operation areformed of a semiconductor other than wide-bandgap semiconductors, lowercosts can be achieved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram showing the configuration of a converteraccording to a first embodiment of the present invention.

FIG. 2 is a time chart showing the waveform of a PWM signal forcontrolling the transistor shown in FIG. 1.

FIG. 3 is a time chart for explaining switching loss of the transistorshown in FIG. 1.

FIG. 4 is a block diagram showing the configuration of the semiconductormodule included in the converter shown in FIG. 1.

FIG. 5 is a circuit block, diagram showing the configuration of anuninterruptible power supply device including the converter shown inFIG. 3.

FIG. 6 is a circuit diagram showing the configuration of an inverterincluded in an uninterruptible power supply device according to a secondembodiment of the present invention.

FIG. 7 is a time chart showing the waveforms of four PWM signals torcontrolling tour transistors shown in FIG. 6.

FIG. 8 is a circuit diagram for explaining current flowing through theinverter shown in FIG. 6.

FIG. 9 is a time chart showing current flowing through the invertershown in FIG. 6.

FIG. 10 is a block diagram showing the configuration of thesemiconductor module included m the inverter shown in FIG. 6.

FIG. 11 is a circuit diagram showing the configuration of an inverteraccording to a third embodiment of the present invention.

FIG. 12 is a circuit diagram showing the configuration of an inverteraccording to a fourth embodiment of the present invention.

FIG. 13 is a circuit block diagram showing the configuration of anuninterruptible power supply device according to a fifth embodiment ofthe present invention.

FIG. 14 is a circuit diagram showing the configuration of the converterand the inverter shown in FIG. 13.

FIG. 15 is a circuit diagram showing the configuration of thebidirectional chopper shown in FIG. 13.

DESCRIPTION OF EMBODIMENTS First Embodiment

FIG. 1 is a circuit diagram showing the configuration of a converteraccording to a first embodiment of the present invention. In FIG. 1,this converter includes an input terminal T0, output terminals T1 to T3,diodes D1 to D6, and a transistor Q1.

The input terminal T0, for example, receives AC voltage VAC of acommercial frequency from a commercial AC power source 10. Outputterminals T1, T3 are connected with the positive electrode and thenegative electrode of a battery B1, respectively. Output terminals T3,T2 are connected with the positive electrode and the negative electrodeof a battery B2, respectively. Each of batteries B1, B2 stores DC power.Batteries B1 and B2 are charged with DC voltage of the same value.

When the voltages at output terminals T1, T2, T3 are DC voltages V1, V2,V3, respectively, V1>V3>V2 holds, which can be written intoV3=(V1+V2)/2. This converter converts AC voltage VAC applied to inputterminal T0 into DC voltages V1 to V3 to be output to output terminalsT1 to T3, respectively. If output terminal T3 is grounded, DC voltagesV1 to V3 are to be positive voltage, negative voltage, and 0 V,respectively.

Each of diodes D1, D2 is a Schottky barrier diode formed of SiC (siliconcarbide) which is a wide-bandgap semiconductor. The rated current ofeach diode D1, D2 is, for example, 600 A and is larger than the ratedcurrent of each of diodes D3 to D6 and transistor Q1.

Each of diodes D3 to D6 is formed of Si (silicon) which is asemiconductor other than wide-bandgap semiconductors. The rated currentof each of diodes D3 to D6 is, for example, 450 A.

Transistor Q1 is an n-channel MOS transistor formed of SIC (siliconcarbide) which is a wide-bandgap semiconductor. The rated current oftransistor Q1 is, for example, 500 A.

The reason why the specifications of diodes D1, D2, the specificationsof diodes D3 to D6, and the specifications of transistor Q1 differ asdescribed above will be described later.

Diode D1 has its anode connected to input terminal T0 and its cathodeconnected to output terminal T1 (first output terminal). Diode D2 hasits anode connected to output terminal T2 (second output terminal) andits cathode connected to input terminal T0.

Diodes D3, D4 have their anodes connected to input terminal T0 andoutput terminal T3 (third output terminal), respectively, and theircathodes connected to each other. Diodes D5, D6 have their cathodesconnected to input terminal T0 and output terminal T3, respectively; andtheir cathodes connected to each other.

Transistor Q1 has its drain (first electrode) connected to the cathodesof diodes D3, D4 and its source (second electrode) connected to thecathodes of diodes D5, D6. Diodes D3 to D6 and transistor Q1 constitutea first bidirectional switch connected between input terminal T0 andoutput terminal T3.

The operation of this converter will now be described. A PWM (pulsewidth modulation) signal φ is applied from a control device (not shown)to the gate of transistor Q1. FIGS. 2(a) to (d) are diagrams showing thegeneration method and the waveform of PWM signal φ1. In particular, FIG.2(a) shows the waveforms of sine-wave command value signal CM,positive-side triangular wave carrier signal CA1, and negative-sidetriangular wave carrier signal CA2, and FIGS. 2(b) to (d) show thewaveforms of PWM signals φ1B, φ1, respectively.

In FIGS. 2(a) to (d), the frequency of sine-wave command value signal CMis, for example, a commercial frequency. The phase of sine-wave commandvalue signal CM is, for example, the same as the phase of AC voltage VACof a commercial frequency. Carrier signals CA1, CA2 have the same cycleand phase. The cycle of carrier signals CA1, CA2 is sufficiently smallerthan the cycle of sine-wave command value signal CM.

The level of sine-wave command value signal CM is compared with thelevel of positive-side triangular wave carrier signal CA1. When thelevel of sine-wave command value signal CM is higher than the level ofpositive-side triangular wave carrier signal CA1, PWM signal φ1B is setto “L” level. When the level of sine-wave command value signal CM islower than the level of positive-side triangular wave carrier signalCA1, PWM signal φ1B is set to “H” level.

Therefore, in the period during which the level of sine-wave commandvalue signal CM is positive, PWM signal φ1B is set to “H” level and “L”level in synchronization with carrier signal CA1, and in the periodduring which the level of sine-wave command value signal CM is negative,PWM signal φ1B is fixed to “H”level.

The level of sine-wave command value signal CM is compared with thelevel of negative-side triangular wave carrier signal CA2. When thelevel of sine-wave command value signal CM is higher than the level ofnegative-side triangular wave carrier signal CA2, PWM signal φ1A is setto “H” level. When the level of sine-wave command value signal CM islower than the level of negative-side triangular wave carrier signalCA2, PWM signal φ1A is set to “L” level.

Therefore, in the period during which the level of sine-wave commandvalue signal CM is positive, PWM signal φ1A is set to “H” level. In theperiod during which the level of sine-wave command value signal CM isnegative, PWM signal φ1A is set to “H” level and “L” level insynchronization with carrier signal CA2. PWM signal φ1 is the AND signalof PWM signals φ1A, φ1B. PWM signal φ1 is set to “H” level and “L” levelin synchronization with carrier signals CA1, CA2.

The ratio between the time during which PWM signal is set to “H” levelin one cycle and the time of one cycle of PWM signal is called dutyratio. In the period during which the level of sine-wave command valuesignal CM is positive, the duty ratio of PWM signal φ1 is smallest inthe vicinity of the positive peak (90 degrees) of sine-wave commandvalue signal CM, increases farther from the peak, and is largest in thevicinity of 0 degree and is the vicinity of 180 degrees. In the periodduring which the level of sine-wave command value signal CM is negative,the duty ratio of PWM signal φ1 is smallest k the vicinity of thenegative peak (270 degrees) of sine-wave command value signal CM,increases farther from the peak, and is smallest in the vicinity of 180degrees and in the vicinity of 360 degrees.

Current flowing through each of diodes D1 to D6 and transistor Q1 duringoperation of the converter will now be described. It is assumed that thepower factor is 1.0, and sine-wave command value signal CM and ACvoltage VAC match in phase. In the period during which the level ofsine-wave command value signal CM is positive, when transistor Q1 isturned off, current I1 at a level in accordance with the level of ACvoltage VAC flows from input terminal T0 through diode D1 to outputterminal T1, and when transistor Q1 is turned on, current I1A at a levelthat complements current I1 flows in a path from input terminal T0through diode D3, transistor Q1, and diode D6 to output terminal T3.

In this period, the effective value of current flowing through diode D1is largest among diodes D1 to D6 and transistor Q1, and switching lossoccurs in transistor Q1. Every time transistor Q1 changes from the offstate to the on state, reverse bias voltage is applied to diode D1, anddiode D1 performs reverse recovery operation. In this period, no currentflows through diodes D2, D4, D5.

In the period during which the level of sine-wave command value signalCM is negative, when transistor Q1 is turned off current I2 at a levelin accordance with the level of AC voltage VAC flows from outputterminal T2 through diode D2 to input terminal T0, and when transistorQ1 is turned on, current I2A at a level that complements current I2flows in the path from output terminal T3 through diode D4, transistorQ1, and diode D5 to input terminal T0.

In this period, the effective value of current flowing through diode D2is largest among diodes D1 to D6 and transistor Q1, and switching lossoccurs in transistor Q1. Every time transistor Q1 changes from the offstate to the on state, reverse bias voltage is applied to diode D2, anddiode D2 performs reverse recovery operation. In this period, no currentflows through diodes D1, D3, D6.

In summary, large current, flows through diodes D1, D2, and diodes D1,D2 perform reverse recovery operation. Current smaller than the one indiodes D1, D2 flows through diodes D3 to D6, and diodes D3 to D6 do notperform reverse recovery operation. Current flows through transistor Q1,and switching loss occurs in transistor Q1.

Thus, as described above, diodes D1, D2 are formed of SIC which is awide-bandgap semiconductor, and Schottky barrier diodes with ratedcurrent of a large value (for example, 600 A) are employed, therebyreducing the recovery loss during reverse recovery operation. Diodes D3to D6 are formed of Si which is a semiconductor other than wide-bandgapsemiconductors, and diodes with rated current of a small value (forexample, 450 A) are employed, thereby reducing the costs.

Furthermore, transistor Q1 is formed of SiC which is a wide-bandgapsemiconductor, and an n-channel MOS transistor with rated current of alarge value (for example, 500 A) is employed, thereby reducing switchingloss.

FIG. 3(a) is a time chart showing switching operation of an n-channelMOS transistor (referred to as Si transistor) formed of Si, and FIG.3(b) is a time chart showing switching operation of an n-channel MOStransistor (referred to as SiC transistor) formed of SiC.

In FIGS. 3(a)(b), it is assumed that in the initial state, the gatesignal (not shown) is set to “H” level to turn on the transistor,constant current I flows through the transistor, and drain-sourcevoltage Vds is 0 V. When the gate signal is lowered from “H” level to“L” level to turn oil the transistor at a certain time, current Idecreases and voltage Vds increases.

As can be understood from FIGS. 3(a)(b), time Ta taken for current I tostart dropping to reach 0 A in the Si transistor is longer than time Tbtaken for current I to start dropping to reach 0 A in the SiCtransistor. In the Si transistor, current I is reduced quickly up to acertain value but it takes time to reach 0 A from the certain value.Current that flows from a certain value up to 0 A is called tailcurrent.

By contrast, in the SiC transistor, current I decreases quickly and aslight overshoot occurs. The switching loss of the transistor is theproduct of current I and voltage Vds and corresponds to the area of thehatched section in the figure. Therefore, the switching loss of the SiCtransistor is smaller than the switching loss of the Si transistor.

FIG. 4 is a diagram showing the appearance of the converter shown inFIG. 1. In FIG. 4, the converter includes one semiconductor module M1.In the inside of semiconductor module M1, diodes D1 to D4 and transistorQ1 are provided. On the outside of semiconductor module M1, inputterminal T0 and output terminals T1 to T3 are provided. Furthermore, onthe outside of semiconductor module M1, a signal terminal for applyingPWM signal φ1 to the gate of transistor Q1 is provided, though not shewnin the figure for simplicity of the figure.

FIG. 5 is a circuit block diagram showing the configuration of anuninterruptible power supply device including the converter shown inFIG. 1. In FIG. 5, the uninterruptible power supply device Includes aninput filter 1, a converter 2, a DC positive bus L1, a DC negative busL2, a DC neutral point bus L3, capacitors C1, C2, an inverter 3, anoutput filter 4, a bidirectional chopper 5, and a control device 6.

Input filter 1 is a low pass filter and allows AC power of a commercialfrequency from a commercial AC power source 10 so pass through inputterminal T0 of converter 2 and prevents the signal of a carrierfrequency produced in converter 2 from passing toward commercial ACpower source 10.

DC positive bus L1, DC negative bus L2, and DC neutral point bus L3 havetheir one ends connected to output terminals T1, T2, T3 of converter 2,respectively, and their other ends connected to three input terminals ofinverter 3, respectively. Capacitor C1 is connected between buses L1 andL3, and capacitor C2 is connected between buses L3 and L2. Buses L1, L3axe connected to the positive electrode and the negative electrode ofbattery B1, respectively, and buses L3, L2 are connected to the positiveelectrode and the negative electrode of battery B2, respectively.

As shown in FIG. 1, converter 2 includes input terminal T0, outputterminals T1 to T3, diodes D1 to D6, and transistor Q1 and is controlledby PWM signal φ1 from control device 5.

During normal operation in which AC power is supplied normally fromcommercial AC power source 10, converter 2 converts AC power suppliedfrom commercial AC power source 10 through input filter 1 into DC powerand supplies the DC power to each of batteries B1, B2 and also toinverter 3. Each of batteries B1, B2 stores DC power.

In other words, converter 2 is controlled by PWM signal φ1 applied fromcontrol device 5, generates DC voltages V1 to V3 based on AC voltage VACsupplied from commercial AC power source 10 through input filter 1, andapplies the generated DC voltages V1 to V3 to DC positive bus L1, DCnegative bus L2, and DC neutral point bus L3, respectively. If outputterminal 13 is grounded, DC voltages V1 to V3 are to be positivevoltage, negative voltage, and 0 V, respectively. DC voltages V1 to V3are smoothed by capacitors C1, C2 DC voltages V1 to V3 are supplied tobatteries B1, B2 and inverter 3. During a power failure in which thesupply of AC power from commercial AC power source 10 is stopped,transistor Q1 is fixed to the state and the operation of converter 2 isstopped.

During normal operation in which AC power is supplied normally fromcommercial AC power source 10, inverter 3 converts DC power generated inconverter 2 into AC power. During a power failure in which the supply ofAC power from commercial AC power source 10 is stopped, inverter 3converts DC power of batteries B1, B2 into AC power.

In other words, inverter 3 generates AC voltage in three levels based onDC voltages V1 to V3 supplied from converter 2 through buses L1 to L3during normal operation and generates AC voltage in three levels basedon DC voltages V1 to V3 supplied from batteries B1, B2 through buses L1to L3 during a power failure.

Output, filter 4 is connected between the output terminal of inverter 3and load 11. Output filter 4 is a low pass filter and allows AC power ofa commercial frequency, of AC power output from inverter 3, to passthrough load 11 and prevents the signal of a carrier frequency generatedin inverter 3 from passing toward load 11. In other words, output filter4 converts the output, voltage of inverter 3 into a sine wave of acommercial frequency to be supplied to load 11.

Control device 5 controls converter 2 and inverter 3 by supplying FWMsignal while monitoring AC voltage from commercial AC power source 10,AC voltage output to load 11, DC voltages V1 to V3, and the like,

The operation of this uninterruptible power supply device will now bedescribed. During normal operation in which AC power is suppliednormally from commercial AC power source 10, AC power from commercial ACpower source 10 is supplied to converter 2 through input filter 1 andconverted into DC power by converter 2. The DC power generated byconverter 2 is stored into batteries B1, B2 and also supplied toinverter 3 and converted by inverter 3 into AC power of a commercialfrequency. The AC power generated by inverter 3 is supplied to load 11through output filter 4 to bring load 11 into operation.

During a power failure in which the supply of AC power from commercialAC power source 10 is stopped, the operation of converter 2 is stopped,and DC power of batteries B1, B2 is supplied to inverter 3 and convertedinto AC power of a commercial frequency by inverter 3. The AC powergenerated by inverter 3 is supplied to load 11 through output filter 4to keep load 11 operating.

Therefore, even when a power failure occurs, the operation of load 11continues as long as DC power is stored in batteries B1, B2. When thesupply of AC power from commercial AC power source 10 is resumed, theoperation of converter 2 is resumed, and the DC power generated, inconverter 2 is supplied to batteries B1, B2 and inverter 3 to restorethe original state.

As described above, in the present first embodiment, since a converteris constituted with one transistor Q1 and sis diodes D1 to D6, it ispossible to reduce the number of semiconductor devices compared with theconventional one to achieve size reduction and lower costs of thedevice. Moreover, since diodes D1, D2 performing reverse recoveryoperation and transistor Q1 switching current are formed of awide-bandgap semiconductor, recovery loss and switching loss can bereduced, In addition, since diodes D3 to D6 that do not perform reverserecovery operation are formed of a semiconductor other than wide-bandgapsemiconductors, lower costs can be achieved.

Although SiC is used as a wide-bandgap semiconductor in the presentfirst embodiment, the present invention is not limited thereto and anyother semiconductor can be used as long as it is a wide-bandgapsemiconductor, for example, GaN (gallium nitride) may be used as awide-bandgap semiconductor.

Second Embodiment

FIG. 6 is a circuit block, diagram showing the configuration of inverter3 included in an uninterruptible power supply device according to asecond embodiment of the present invention. The overall configuration ofthe uninterruptible power supply device is as shown in FIG. 5. Converter2 included in the uninterruptible power supply device is the convertershown in FIG. 1. In FIG. 6, this inverter 3 includes input terminals T11to T13 (first to third output terminals), an output terminal T14 (firstto third output terminals), transistors Q11 to Q14 (second to fifthtransistors), and diodes D11 to D14 (seventh to tenth diodes).

Input terminals T11 to T13 are connected to DC positive bus L1, DCnegative bus L2 and DC neutral point bus L3 in FIG. 5, respectively.Input terminals T11, T13 are connected with the positive electrode andthe negative electrode of battery B1, respectively. Input terminals T13,T12 are connected with the positive electrode and the negative electrodeof battery B2, respectively. Each of batteries B1, B2 outputs DCvoltage. The output voltage of battery B1 is equal to the output voltageof battery B2. Therefore, DC voltages V1, V2, V3 are applied to inputterminals T11, T12, T13, respectively, where V1>V3>V2, which can bewritten into V3−(V1+V2)/2. This inverter is to convert DC voltages V1 toV3 applied to input terminals T11 to T13 into AC voltage V4 in threelevels to be output to output terminal T14. If input terminal T13 isgrounded, DC voltages V1 to V3 are to be positive voltage, negativevoltage, and 0 V, respectively.

Each of transistors Q11, Q12 is an n-channel MOS transistor formed ofSiC (silicon carbide) which is a wide-bandgap semiconductor. The ratedcurrent of each of transistors Q11, Q12 is for example, 600 A and largerthan the rated current of each of transistors Q13, Q14 and diodes D11 toD14.

Each of transistors Q13, Q14 is an IGBT (insulated Gate BipolarTransistor) formed of Si (silicon) which is a semiconductor other thanwide-bandgap semiconductors. The rated current of each of transistorsQ13, Q14 is, for example, 450 A.

Each of diodes D11, D12 is formed of Si (silicon) which is asemiconductor other than wide-bandgap semiconductors. The rated currentof each of diodes D11, D12 is, for example, 300 A.

Each of diodes D13, D14 is a Schottky barrier diode formed of SiC(silicon carbide) which is a wide-bandgap semiconductor. The ratedcurrent of each of diodes D13, D14 is, for example, 500 A. The ratedcurrent of transistors Q11, Q12 is larger than the rated current of eachof transistors Q13, Q14 and diodes D11 to D14.

The reason why the specifications of transistors Q11, Q12 differ fromthe specifications of transistors Q13, Q14 and the specifications ofdiodes D11, D12 differ from the specifications of diodes D13, D14 willbe described later.

Transistor Q11 has its drain (first electrode) connected to inputterminal T11 (first output terminal) and its source (second electrode)connected to output terminal T14 (fourth output terminal). Diode D11 hasits anode connected to output terminal T14 and its cathode connected toinput terminal T11.

Transistor Q12 has its drain connected to output terminal T14 and itssource connected to input terminal T12 (second output terminal). DiodeD6 has its anode connected to input terminal T12 and its cathodeconnected to output terminal T14. That is, diodes D11, D12 are connectedin anti-parallel with transistors Q11, Q12, respectively.

Transistors Q13, Q14 have their collectors (first electrodes) connectedto each other, and transistors Q13, Q14 have their emitters (secondelectrodes) connected to input terminal T13 (third output, terminal) andoutput terminal T14, respectively. Diodes D13, D14 have their cathodesconnected together to the collectors of transistors Q13, Q14 and theiranodes connected to input terminal T13 and output terminal T14,respectively. That is, diodes D13, D14 are connected in anti-parallelwith transistors Q13, Q14, respectively. Transistors Q13, Q14 and diodesD13, D14 constitute a second bidirectional switch connected betweeninput terminal T13 and output terminal T14.

The operation of this inverter will now be described. PWM signals φ11 toφ14 are applied from control device 5 to the gates of transistors Q11 toQ14, respectively. FIGS. 7(a) to (e) are diagrams showing the generationmethod and the waveforms of PWM signals φ1 to φ14. In particular, FIG.7(a) shows the waveforms of sine-wave command value signal CM,positive-side triangular wave carrier signal CA1, and negative-sidetriangular wave carrier signal CA2, and FIGS. 7(b) to (e) show thewaveforms of PWM signals φ11, φ14, φ13, φ12, respectively.

In FIGS. 7(a) to (e), the frequency of sine-wave command value signal CMis, for example, a commercial frequency. Carrier signals CA1, CA2 havethe same cycle and phase. The cycle of carrier signals CA1, CA2 issufficiently smaller than the cycle of sine-wave command value signalCM.

The level of sine-wave command value signal CM is compared with thelevel of positive-side triangular wave carrier signal CA1. When thelevel of sine-wave command value signal CM is higher than the level ofpositive-side triangular wave carrier signal CAT, PWM signals φ11, φ13are set to “H” level and “L” level, respectively. When the level ofsine-wave command value signal CM is lower than the level ofpositive-side triangular wave carrier signal CA1, PWM signals φ11, φ13are set to “L” level, and “H” level, respectively.

Therefore, in the period during which the level of sine-wave commandvalue signal CM is positive, PWM signals φ11 and φ3 are alternately setto “H” level in synchronization with carrier signal CA1, and transistorsQ11 and Q13 are alternately turned on. In the period during which thelevel of sine-wave command value signal CM is negative, PWM signals φ11,φ3 are fixed to “L” level and “H” level, respectively, transistor Q11 isfixed to the off state, and transistor Q13 is fixed to the on state.

The level of sine-wave command value signal CM is compared with thelevel of negative-side triangular wave carrier signal CA2. When thelevel of sine-wave command value signal CM is higher than the level ofnegative-side triangular wave carrier signal CA2, PWM signals φ12, φ4are set to “L” level and “H” level, respectively. When the level ofsine-wave command value signal CM is lower than the level ofnegative-side triangular wave carrier signal CA2, PWM signals φ12, φ14are set to “H” level and “L” level, respectively.

Therefore, in the period during which the level of sine-wave commandvalue signal CM is positive, PWM signals φ12, φ14 are fixed to “L” leveland “H” level, respectively, transistor Q12 is fixed to the off state,and transistor Q14 is fixed to the on state. In the period during whichthe level of sine-wave command value signal CM is negative, PWM signalsφ12 and φ14 are alternately set to “H” level, in synchronization withcarrier signal CA2, and transistors Q12 and Q14 are alternately turnedon.

The ratio between the time during which PWM signal is set to “H” levelin one cycle and the time of one cycle of FWM signal is called dutyratio. In the period during which the level of sine-wave command valuesignal CM is positive, the duty ratio of FWM signal φ11 is largest inthe vicinity of positive peak (90 degrees) of sine-wave command valuesignal CM, decreases farther from the peak, and is zero in the vicinityof 0 degree and in the vicinity of 180 degrees. In the period duringwhich the level, of sine-wave command value signal CM is negative, theduty ratio of PWM signal φ11 is fixed to zero. PWM signal φ13 is acomplementary signal to PWM signal φ11.

The duty ratio of PWM signal φ12 is fixed to zero in the period duringwhich the level of sine-wave command value signal CM is positive. Theduty ratio of PWM signal φ12 is largest in the vicinity of the negativepeak (270 degrees) of sine-wave command value signal CM, decreasesfarther from the peak, and is zero in the vicinity of 180 degrees and inthe vicinity of 360 degrees. The duty ratio of PWM signal φ12 is fixedto zero in the period during which the level of sine-wave command valuesignal CM is positive. PWM signal φ14 is a complementary signal to PWMsignal φ12.

Next, current flowing through each of transistors Q11 to Q14 and diodesD11 to D14 during operation of the inverter will be described. As shownin FIG. 8, current flowing from input terminal T11 to output terminalT14 is denoted as I11, current flowing from output terminal T14 to inputterminal T12 is denoted as I12, current flowing from input terminal T13to output terminal T14 is denoted as I13, and current flowing fromoutput terminal T14 to input terminal T13 is denoted as I14.

FIGS. 9(a) to (i) are time charts showing the operation of the inverter.In particular. FIG. 9(a) shows the waveforms of sine-wave command valuesignal CM, positive-side triangular wave carrier signal CA1, andnegative-side triangular wave carrier signal CA2, FIGS. 9(b)(d)(f)(h)are the waveforms of PWM signals φ11, φ14, φ13, φ12, respectively, andFIGS. 9(c)(e)(g)(i) show the waveforms of currents I11, I14, I13, I12,respectively. Of currents I11 to I14, the positive current shows currentflowing through transistor Q, and the negative current shows currentflowing through diode D. The case where the power factor is 1.0 isshown.

In FIGS. 9(a) to (i), in the period during which the level of sine-wavecommand value signal CM is positive, PWM signals φ14, φ12 are fixed to“H” level and “L” level, respectively, and PWM signals φ11 and φ13 arealternately set to “H” level. Therefore, transistors Q14, Q12 are fixedto the on state and the off state, respectively, transistors Q11 and Q13are alternately turned on, and DC voltages V1 and V3 alternately appearat output terminal T14.

In this period, when transistor Q11 is turned on, current I11 at a levelin accordance with the on time of transistor Q11 flows, and whentransistor Q11 is turned off, current I13 at a level that complementscurrent I11 flows in the path of diode D13 and transistor Q14.

Since transistor Q12 is fixed to the off state, no current flows throughtransistor Q12, and switching loss does not occur in transistor Q12.Although transistor Q13 is turned on/off current flows through diodeD13, and no current flows through transistor Q13. Then, switching lossdoes not occur in transistor Q13. Since transistor Q14 is fixed to theon state, current flows through transistor Q14 but switching loss doesnot occur in transistor Q14. Therefore, in this period, the effectivevalue of current flowing through transistor Q11 is largest amongtransistors Q11 to Q14, and switching loss in transistor Q11 is largest.

Every time transistor Q11 changes from the off state to the on state,reverse bias voltage is applied to diode D13, and diode D13 performsreverse recovery operation. In this period, no current flows in otherdiodes D11, D12, D14.

In the period during which the level of sine-wave command value signalCM is negative, PWM signals φ13, φ11 are fixed to “H” level and “L”level, respectively, and PWM signals φ12 and φ14 are alternately set to“H” level. Therefore, transistors Q13, Q11 are fixed to the on state andthe off state, respectively, transistors Q12 and Q14 are alternatelyturned on, and DC voltages V2 and V3 alternately appear at outputterminal T14.

In this period, when transistor Q12 is turned on, current I12 at a levelin accordance with the on time of transistor Q12 flows, and whentransistor Q12 is turned off; current I13 flows in the path of diode D14and transistor Q13.

Since transistor Q11 is fixed to the off stale, no current flows throughtransistor Q11, and switching loss does not occur in transistor Q11.Although transistor Q14 is turned on/off current flows through diodeD12, and no current flows through transistor Q14. Then, switching lossdoes not occur in transistor Q14. Since transistor Q13 is fixed to theon state, current flows through transistor Q13, but switching loss doesnot occur in transistor Q13. Therefore, in this period, the effectivevalue of current flowing through transistor Q12 is largest amongtransistors Q11 to Q14, and the switching loss in transistor Q12 islargest.

Furthermore, every time transistor Q12 changes from the off state to theon state, reverse bias voltage is applied to diode D14, and diode D14performs reverse recovery operation. In this period, no current flows inother diodes D11, D12, D13.

In summary, large current flows through transistors Q11, Q12, andswitching loss occurs in transistors Q11, Q12. Current smaller than intransistors Q11, Q12 flows through transistors Q13, Q14, and switchingloss does not occur in transistors Q13, Q14.

Thus, as described above, transistors Q11, Q12 are formed of SiC whichis a wide-bandgap semiconductor, and n-channel MOS transistors withrated current of a large value (for example, 600 A) are employed,thereby reducing switching loss. Furthermore, transistors Q13, Q14 areformed of Si which is a semiconductor other than wide-bandgapsemiconductors, and IGBTs with rated current of a small value (forexample, 450 A) are employed, thereby achieving lower costs.

Current equivalent to that of transistors Q13, Q14 flows through diodesD13, D14, and diodes D13, D14 perform reverse recovery operation Nocurrent flows through diodes D11, D12. It is noted that diodes D11, D12are provided to protect transistors Q11, Q12 from voltage produced in aninductor when the inductor is used as a load.

Thus, as described above, diodes D13, D14 are formed of SiC which is awide-bandgap semiconductor, and Schottky barrier diodes with ratedcurrent having a value (for example, 500 A) equivalent to that oftransistors Q13, Q14 are employed, thereby reducing recovery loss duringreverse recovery operation. Diodes D11, D12 are formed of Si which is asemiconductor other than wide-bandgap semiconductors, and diodes withrated current having a small value (for example, 300 A) are employed,thereby achieving lower costs.

FIG. 10 is a diagram showing the appearance of inverter 3 shown in FIG.6. In FIG. 10, inverter 3 includes one semiconductor module M2. In theinside of semiconductor module M2, transistors Q11 to Q14 and diodes D11to D14 are provided. On the outside of semiconductor module M2, inputterminals T11 to T13 and output terminal T14 are provided. Furthermore,on the outside of semiconductor module M2, four signal terminals forapplying PWM signals φ1 to φ14 to the gates of transistors Q11 to Q14are provided, though the four signal terminals are not shown in thefigure for simplicity of the figure.

As described above, in the present, second embodiment, since n-channelMOS transistors formed of a wide-bandgap semiconductor are used astransistors Q11, Q12 that turn on/off current, and IGBTs formed of asemiconductor other than wide-bandgap semiconductors are used astransistors QB13, Q14 that do not turn on/off current, reduction ofswitching loss and lower costs can be achieved.

Furthermore, since Schottky barrier diodes formed of a wide-bandgapsemiconductor are used as diodes D13, D14 that perform reverse recoveryoperation and diodes formed of a semiconductor other than wide-bandgapsemiconductors are used as diodes D11, D12 that do not perform reverserecovery operation, reduction of recovery loss and lower costs can beachieved.

Although SiC is used as a wide-bandgap semiconductor in the presentsecond embodiment, the present invention is not limited thereto and anysemiconductor can be used as long as it is a wide-bandgap semiconductor.For example, GaN (gallium nitride) may be used as a wide-bandgapsemiconductor.

Third Embodiment

FIG. 11 is a circuit diagram showing a configuration of an inverteraccording to a third embodiment of the present invention, in comparisonwith FIG. 6. Referring to FIG. 11, this inverter differs from inverter 3in FIG. 6 in that the parallel connection structure of transistor Q13and diode D13 and the parallel connection structure of transistor Q14and diode D14 are replaced.

Transistors Q13, Q14 have their emitters connected to each other andtheir collectors connected to input terminal T13 and output terminalT14, respectively. Transistors Q11 to Q14 are controlled by PWM signalsφ1 to φ14, respectively. When DC voltages V1, V3 are alternately outputto output terminal T14, transistor Q14 is turned on, and transistorsQ11, Q13 are alternately turned on. When DC voltages V2, V3 arealternately output to output terminal T14, transistor Q13 is turned onand transistors Q12, Q14 are alternately turned on.

Other configuration and operation are the same as in the secondembodiment and a description thereof will not be repeated. The presentthird embodiment also achieves the same effects as the secondembodiment.

Fourth Embodiment

FIG. 12 is a circuit diagram showing a configuration of an inverteraccording to a fourth embodiment of the present invention, in contrastwith FIG. 6. Referring to FIG. 12, this inverter differs from inverter 3in FIG. 6 in that the collectors of transistors Q13, Q14 are separatedfrom the cathodes of diodes D13, D14, the collector of transistor Q13 isconnected to the cathode of diode D14, and the collector of transistorQ14 is connected to the cathode of diode D13.

Transistors Q11 to Q14 are controlled by PWM signals φ11 to φ14,respectively. When DC voltages V1, V3 are alternately output to outputterminal T14, transistor Q14 is turned on and transistors Q11, Q13 arealternately turned on. When DC voltages V2, V3 are alternately output tooutput terminal T14, transistor Q13 is turned on and transistors Q12,Q14 are alternately turned on.

Other configuration and operation are the same as in the secondembodiment and a description thereof will not be repeated. The presentfourth embodiment also achieves the same effects as the secondembodiment.

Fifth Embodiment

FIG. 13 is a circuit block diagram showing the configuration of anuninterruptible power supply device according to a fifth embodiment ofthe present invention. FIG. 14 is a. circuit diagram showing theconfiguration of converter 22 and inverter 24 shown in FIG. 13. FIG. 15is a circuit diagram showing the configuration of bidirectional chopper23 shown in FIG. 13. In FIG. 13 to FIG. 15, the uninterruptible powersupply device includes an input filter 21, a converter 22, a DC positivebus L1, a DC negative bus L2, a DC neutral point bus L3, capacitors C1,C2, a bidirectional chopper 23, an inverter 24, and an output filter 25.For simplicity of the figure, the control device controlling converter22, bidirectional chopper 23, and inverter 24 is not shown.

Input filter 21 includes reactors 31 to 33 and capacitors 34 to 36.Reactors 31 to 33 have their one terminals receiving three-phase ACvoltages VU, VV, VW, respectively, from commercial AC power source 20have their other terminals connected to input terminals T0 a to T0 c ofconverter 22. Capacitors 34 to 36 have one electrodes connected to oneterminals of reactors 31 to 33, respectively; and their other electrodesconnected together to a neutral point NP. Reactors 31 to 33 andcapacitors 34 to 36 constitute a low pass filter. Input filter 21 allowsthree-phase AC power of a commercial frequency from commercial AC powersource 20 to pass through converter 22 and prevents a signal of acarrier frequency produced in converter 22 from passing towardcommercial AC power source 20.

DC positive bus L1, DC negative bus L2, and DC neutral point bus L3 havetheir one ends connected to output terminals T1, T2, T3 of converter 22,respectively, and their other ends connected to input terminals T11,T12, T13 of inverter 24, respectively. Capacitor C1 is connected betweenbuses L1 and L3, and capacitor C2 is connected between buses L3 and L2.Buses L1 to L3 are connected to battery B11 through bidirectionalchopper 23.

As shown in FIG. 14, converter 22 includes input terminals T0 a to T0 c,output terminals T1 to T3, diodes D1 a to D1 e, D2 a to D2 c, andbidirectional switches S1 a to S1 c. Input terminals T0 a to T0 creceive three-phase AC voltages VU, VV, VW, respectively, supplied fromcommercial AC power source 20 through input filter 21. Diodes D1 a to D1e have their anodes connected to input terminals T0 a to T0 c,respectively, and their cathodes connected together to output terminalT1. Diodes D2 a to D2 c have their anodes connected together to outputterminal T2 and their cathodes connected to input terminals T0 a to T0c, respectively.

Bidirectional switches S1 a to S1 c have their one terminals connectedto input terminals T0 a to T0 c, respectively, and their other terminalsconnected together to output terminal T3. Each of bidirectional switchesS1 a to S1 c includes diodes D3 to D6 and n-channel MOS transistor Q1 asshown in FIG. 1.

The anode of diode D3 and the cathode of diode D5 are connected togetherto input terminal T0 a (or T0 b, or T0 c). The anode of diode D4 and thecathode of diode D6 are connected together to output terminal T3. Thecathodes of diodes D3, D4 are connected to each other, and the anodes ofdiodes D5, D6 are connected to each other. The drain of transistor Q1 isconnected to the cathodes of diodes D3, D4, and the source of transistorQ1 is connected to the anodes of diodes D5, D6.

Transistors Q1 in bidirectional switches S1 a to S1 c are controlled byPWM signals φ1 a, φ1 b, φ1 c, respectively, from the control device (notshown). The waveforms of FWM signals φ1 a, φ1 b, φ1 c are similar tothat of PWM signal φ1 shown in FIG. 2(d). The phases of PWM signals φ1a, φ1 b, φ1 c are sychronized with the phases of three-phase AC voltagesVU, VV, VW, respectively, and shifted from each other by 120 degrees.

That is, input terminal T0 a, output terminals T1 to T3, diodes D1 a, D2a, and bidirectional switch S1 a constitute the converter shown in FIG.1 to convert AC voltage VU into DC voltages V1 to V3 to be output tooutput terminals T1 to T3. Input terminal T0 b, output terminals T1 toT3, diodes D1 b, D2 b, and bidirectional switch S1 b constitute theconverter shown in FIG. 1 to convert AC voltage VV into DC voltages V1to V3 to be output to output terminals T1 to T3. Input terminal T0 c,output terminals T1 to T3, diodes D1 c, D2 c, and bidirectional switchS1 c constitute the converter shown in FIG. 1 to convert AC voltage VWinto DC voltages V1 to V3 to be output to output terminals T1 to T3.Converter 22 converts three-phase AC voltages VU, VV, VW into DCvoltages V1 to V3 to be output to output terminals T1 to T3.

As described in the first embodiment, diodes D1 a to D1 e, D2 a to D2 cand transistors Q1 in bidirectional switches S1 a to S1 c are formed ofa wide-bandgap semiconductor, and diodes D3 to D6 in bidirectional,switches S1 a to S1 c are formed of a semiconductor other thanwide-bandgap semiconductors. The rated current of each of diodes D1 a toD1 c, D2 a to D2 c is larger than the rated current of each of diodes D3to D6 and transistors Q1.

During normal operation in which three-phase AC power is suppliednormally from commercial AC power source 20, converter 22 convertsthree-phase AC power supplied from commercial AC power source 20 throughinput filter 21 into DC power and supplies the DC power to battery B11through bidirectional chopper 23 and to inverter 24. Battery B11 storesDC power.

In other words, converter 20 is controlled by PWM signals φ1 a, φ1 b, φ1c provided from the control device (not shown), generates DC voltages V1to V3 based on three-phase AC voltages VU, VV, VW supplied fromcommercial AC power source 20 through input filter 21, mid applies thegenerated DC voltages V1 to V3 to DC positive bus L1, DC negative busL2, and DC neutral point bus L3, respectively. If output terminal 13 isgrounded, DC voltages V1 to V3 are to be positive voltage, negativevoltage, and 0 V. respectively. DC voltages V1 to V3 are smoothed bycapacitors C1, C2. DC voltages V1 to V3 are supplied to battery B11through bidirectional chopper 23 and to inverter 24. During a powerfailure in which the supply of AC power from, commercial. AC powersource 20 is stopped, transistor Q1 is fixed to the off state, and theoperation of converter 22 is stopped.

When three-phase AC power is supplied from commercial AC power source20, bidirectional chopper 23 supplies DC power from capacitors C1, C2 tobattery B11, and when the supply of three-phase AC power is stopped fromcommercial AC power source 20, that is, during a power failure, DC poweris supplied from battery B11 to capacitors C1, C2.

That is, as shown in FIG. 15, bidirectional chopper 23 includesterminals T21 to T25, transistors Q21 to Q24, diodes D21 to D24, and anormal mode reactor (DC reactor) 40. Terminals T21 to T23 are connectedto DC positive bus L1, DC negative bus L2, and DC neutral point bus L3,respectively. Terminals T24, T25 are connected to the positive electrodeand the negative electrode of battery B11, respectively.

Transistors Q21, Q22 are connected in series between terminals T21 andT23, and transistors Q23, Q24 are connected in series between terminalsT23 and T22. Diodes D21 to D24 are connected in anti-parallel withtransistors Q21 to Q24, respectively. Normal mode reactor 40 includes acoil 41 connected between the node between transistors Q21 and Q22 andterminal T24, and a coil 42 connected between terminal T25 and the nodebetween transistors Q23 and Q24.

Each of transistors Q21 to Q24 is an IGBT formed of Si (silicon) whichis a semiconductor other than wide-bandgap semiconductors. Each ofdiodes D21 to D24 is formed of Si (silicon) which is a semiconductorother than wide-bandgap semiconductors.

When three-phase AC power is supplied from commercial AC power source20, DC power is supplied from capacitors C1, C2 to battery B11 throughbidirectional chopper 23 to charge battery B11. In this case,transistors Q22, Q23 are fixed to the off state, and transistors Q21,Q24 are alternately turned on.

That is, in the first battery charge mode, transistors Q22 to Q24 areturned off and transistor Q21 is turned on. Thus, current flows fromterminal T21 through transistor Q21, coil 41, battery B11, coil 42, anddiode D23 to terminal T23 to discharge capacitor C1 and charge batteryB11.

In the second battery charge mode, transistors Q22, Q23 are turned off,and transistors Q21, Q24 are turned on. Thus, current flows fromterminal T21 through transistor Q21, coil 41, battery B11, coil 42, andtransistor Q24 to terminal T22 to discharge capacitors C1, C2 and chargebattery B11.

In the third battery charge mode, transistors Q21 to Q23 are turned off,and transistor Q24 is turned on. Thus, current flows from terminal T23through diode D22, coil 41, battery B11, coil 42, and transistor Q24 toterminal T22 to discharge capacitor C2 and charge battery B11.

The first battery charge mode and the third battery charge mode arealternately performed. In the period between the first battery chargemode and the third battery charge mode, electromagnetic energy stored incoils 41, 42 is emitted to allow current to flow in the path of diodeD22, coil 41, battery B11, coil 42, and diode D23 to charge battery B11.The second battery charge mode is a mode in which the first batterycharge mode and the third battery charge mode overlap.

When the supply of three-phase AC power from commercial AC power source20 is stopped, DC power is supplied from battery B11 to capacitors C1,C2 through bidirectional chopper 23 to charge capacitors C1, C2. In thiscase, transistors Q21, Q24 are fixed to the off state, and transistorsQ22, Q23 are alternately turned on.

That is, in the first battery discharge mode, transistors Q21, Q23, Q24are turned off and transistor Q22 is turned on. Thus, current flows fromthe positive electrode of battery B11 through, coil 41, transistor Q22,capacitor C2, diode D24, and coil 42 to the negative electrode ofbattery B11 to discharge battery B11 and charge capacitor C2.

In the second battery discharge mode, transistors Q21 to Q24 are turnedoff. Thus, current flows from the positive electrode of battery B11through coil 41, diode D21, capacitors C1, C2, diode D24, and coil 42 tothe negative electrode of battery B11 to discharge battery B11 andcharge capacitors C1, C2.

In the third battery discharge mode, transistors Q21, Q22, Q24 areturned off, and transistor Q23 is turned on. Thus, current flows fromthe positive electrode of battery B11 through coil 41, diode D21,capacitor C1, transistor Q23, and coil 42 to the negative electrode ofbattery B11 to discharge battery B11 and charge capacitor C1.

The first battery discharge mode and the third battery discharge modeare alternately performed. In the period between the first batterydischarge mode and the third battery discharge mode, when the voltagebetween terminals T21 and T22 is lower than the voltage at battery B11,the second battery discharge mode is performed.

As shown in FIG. 14, inverter 24 includes input terminals T11 to T13,output. terminals T14 a to T14 c, transistors Q11 a to Q11 c, Q12 a toQ12 c, diodes D11 a to D11 c, D12 a to D12 c, and bidirectional switchesS2 a to S2 c. Input terminals T11 to T13 are connected to DC positivebus L1, DC negative bus L2, and DC neutral point bus 13, respectively.

Transistors Q11 a to Q11 c have their drains connected together to inputterminal T11 and their sources connected to output terminals T14 a toT14 c, respectively. Transistors Q12 a to Q12 c have their drainsconnected to output terminals T14 a to T14 c, respectively, and theirsources connected together to input terminal T12. Diodes D11 a to D11 c,D12 a to D12 c are connected, in anti-parallel with transistors Q11 a toQ11 e, Q12 a to Q12 c, respectively.

Bidirectional switches S2 a to S2 c have their one terminals connectedtogether to input terminal T13 and their other terminals connected tooutput terminals T14 a to T14 c, respectively. Each of bidirectionalswitches S2 a to S2 c includes transistors Q13, Q14 and diodes D13, D14as shown in FIG. 6.

The collectors of transistors Q13, Q14 are connected to each other, theemitter of transistor Q13 is connected to input terminal T13, and theemitter of transistor Q14 is connected to output terminal T14 a (or T14b, or T14 c). Diodes D13, D14 are connected in anti-parallel withtransistors Q13, Q14, respectively.

Transistors Q11 a to Q11 c are controlled by PWM signals φ11 a, φ11 b,φ11 c from the control device (not shown), respectively. The waveformsof PWM signals φ11 a, φ11 b, φ11 c are similar to that of PWM signal φ11shown in FIG. 7(b). The phases of PWM signals φ11 a, φ11 b, φ11 c aresynchronized with the phases of three-phase AC voltages VU. VV, VW,respectively, and shifted from each other by 120 degrees.

Transistors Q12 a to Q12 c are controlled by PWM signals φ12 a, φ12 b,φ12 c from the control device (not shown), respectively. The waveformsof PWM signals φ12 a, φ12 b, φ12 c are similar to that of PWM signal φ12shown in FIG. 7(c). The phases of PWM signals φ12 a, φ12 b, φ12 c aresynchronized with the phases of three-phase AC voltages VU, VV, VW shownin FIG. 7(c), respectively, and shifted from each other by 120 degrees.

Transistors Q13 in bidirectional switches S2 a to S2 c are controlled byPWM signals φ13 a, φ13 b, φ13 c from the control device (not shown),respectively. As shown in FIG. 7(d), PWM signals φ13 a, φ13 b, φ13 c arecomplementary signals to PWM signals φ11 a, φ11 b, φ11 c, respectively.

Transistors Q14 in bidirectional switches S2 a to S2 c are controlled byPWM signals φ14 a, φ14 b, φ14 c from the control device (not shown),respectively. As shown in FIG. 7(c), PWM signals φ14 a, φ14 b, φ14 c arecomplementary signals to PWM signals φ12 a, φ12 b, φ2 c, respectively.

That is, input terminals T11 to T13, output terminal T14 a, transistorsQ11 a, Q12 a, diodes D11 a, D12 a, and bidirectional, switch S2 aconstitute the inverter shown in FIG. 6 to convert DC voltages V1 to V3into AC voltage V4 a to be output to output terminal T14 a.

Input terminals T11 to T13, output terminal T14 b, transistors Q11 b,Q12 b, diodes D11 b, D12 b, and bidirectional switch S2 b constitute theinverter shown in FIG. 6 to convert DC voltages V1 to V3 into AC voltageV4 b to be output to output terminal T14 b.

Input terminals T11 to T13, output terminal T14 c, transistors Q11 c,Q12 c, diodes D11 e, D12 c, and bidirectional switch S2 c constitute theinverter shown in FIG. 6 to convert DC voltages V1 to V3 into AC voltageV4 e to be output to output terminal T14 c. AC voltages V4 a to V4 cchange in synchronization with three-phase AC voltages VU, VV, VW, andthe phases of AC voltages V4 a to V4 c are shifted from each other by120 degrees.

As explained in the second embodiment, transistors Q11 a to Q11 c, Q12 ato Q12 c and diodes D13, D14 in bidirectional switches S2 a to S2 c areformed of a wide-bandgap semiconductor, and diodes D11 a to D11 c, D12 ato D12 c and transistors Q13, Q14 in bidirectional switches S2 a to S2 care formed of a semiconductor other than wide-bandgap semiconductors.The rated current of each of transistors Q11 a to Q11 c, Q12 a to Q12 cis larger than the rated current of each of transistors Q13, Q14 anddiodes D13, D14.

During normal operation in which three-phase AC power is suppliednormally from commercial AC power source 20, inverter 24 converts DCpower generated m converter 22 into three-phase AC power. During a powerfailure in which the supply of AC power from commercial AC power source20 is stopped, inverter 24 converts DC power supplied from battery B11through bidirectional chopper 23 into three-phase AC power.

In other words, inverter 24 generates three-phase AC voltages V4 a to V4c based on DC voltages V1 to V3 supplied from converter 22 through busesL1 to L3 during normal operation and generates three-phase AC voltagesV4 a to V4 c based on DC-voltages V1 to V3 supplied from battery B11through bidirectional chopper 23 and buses L1 to L3 during a powerfailure.

As shown in FIG. 13, output filter 25 includes reactors 51 to 53 andcapacitors 54 to 56. Reactors 51 to 53 have their one terminalsconnected to output terminals T14 a to T14 c of inverter 24,respectively, and their other terminals connected to load 26. Capacitors54 to 56 have their one electrodes connected to the other terminals ofreactors 51 to 53, respectively, and their other electrodes connectedtogether to neutral point NP. Reactors 51 to 53 and capacitors 54 to 56constitute a low pass filter.

Output filter 25 allows AC power of a commercial, frequency, of AC poweroutput from inverter 24, to pass through load 26 and prevents a signalof a carrier frequency generated in inverter 24 from passing toward load26. In other words, output filter 25 converts output voltages V4 a to V4c of inverter 24 info sine-wave three-phase AC voltages VR, VS, VT of acommercial frequency to be supplied to load 26. Load 26 is driven bythree-phase AC voltages VR, VS, VT.

The control device (not shown) controls converter 22, bidirectionalchopper 23, and inverter 24 by supplying PWM signal while monitoringthree-phase AC voltages VU, VV, VW from commercial AC power source 20,three-phase AC voltages VR, VS, VT output to load 26, DC voltages V1 toV3, the terminal-to-terminal voltage of battery B11, and the like.

The operation of this uninterruptible power supply device will now bedescribed. During normal operation in which three-phase AC power issupplied normally from commercial AC power source 20, the AC power fromcommercial AC power source 20 is supplied to converter 22 through inputfilter 21 and converted into DC power by converter 22. The DC powergenerated in converter 22 is stored into battery B11 throughbidirectional chopper 23, supplied to inverter 24, and converted intothree-phase AC power of a commercial frequency by inverter 24. Thethree-phase AC power generated by inverter 24 is supplied to load 26through output filter 25 to bring load 26 into operation.

During a power failure in which the supply of AC power from commercialAC power source 20 is stopped, the operation of converter 22 is stopped,and DC power of battery B11 is supplied to inverter 24 throughbidirectional chopper 23 and converted into three-phase AC power of acommercial frequency by inverter 24. The three-phase AC power generatedin inverter 24 is supplied to load 26 through output filter 25 to keepload 26 operating.

Therefore, even when a power failure occurs, the operation of load 26continues as long as DC power is stored In battery B11. When the supplyof AC power from commercial AC power source 20 is resumed, the operationof converter 22 is resumed, and DC power generated in converter 22 issupplied to battery B11 through chopper 23 and to inverter 24 to restorethe original state. The present fifth embodiment also achieves the sameeffects as in the first to fourth embodiments.

The embodiments disclosed here should be understood as beingillustrative rather than being limitative in all respects. The scope ofthe present invention is shown not in the foregoing description but inthe claims, and it is intended that all modifications that come withinthe meaning and range of equivalence to the claims are embraced here.

REFERENCE SIGNS LIST

T0, T0 a to T0 c, T11 to T13 input terminal, T1 to T3, T14, T14 a to T14c output terminal, T21 to T25 terminal, Q1, Q11 to Q14, Q11 a to Q11 c,Q12 a to Q12 c, Q21 to Q24 transistor, D1 to D6, D1 a to D1 c, D2 a toD2 c, D11 to D14, D11 a to D11 c, D12 a to D12 c, D21 to D24 diode, B1,B2, B11 battery, M1, M2 semiconductor module, L21 input filter, 2, 22converter, L1 DC positive bus, L2 DC negative bus, L3 DC neutral pointbus, C1, C2, 34 to 36, 54 to 56 capacitor, 3, 24 inverter, 4, 25 outputfilter, 5 control device, 10, 20 commercial AC power source, 11, 26load, 23 bidirectional chopper, 31 to 33, 51 to 53 reactor, S1 a to S1c, S2 a to S2 c bidirectional switch, 40 normal mode reactor, 41, 42coil.

The invention claimed is:
 1. A converter configured to convertalternating-current voltage applied to an input terminal into first tothird direct-current voltages to be output to first to third outputterminals, respectively, comprising: a first diode having an anode and acathode connected to the input terminal and the first output terminal,respectively; a second diode having an anode and a cathode connected tothe second output terminal and the input terminal, respectively; and afirst bidirectional switch connected between the input terminal and thethird output terminal, the first direct-current voltage being higherthan the second direct-current voltage, and the third direct-currentvoltage being an intermediate voltage between the first direct-currentvoltage and the second direct-current voltage, the first bidirectionalswitch including third to sixth diodes and a first transistor, the thirddiode having an anode connected to the input terminal and a cathodeconnected to a first electrode of the first transistor, and the fourthdiode having an anode connected to the third output terminal and acathode connected to the first electrode of the first transistor, thefifth diode having a cathode connected to the input terminal and ananode connected to a second electrode of the first transistor, and thesixth diode having a cathode connected to the third output terminal andan anode connected to the second electrode of the first transistor, thefirst transistor being turned on or off in a predetermined cycle, thefirst diode, the second diode, and the first transistor being eachformed of a wide-bandgap semiconductor, and the third to sixth diodesbeing each formed of a semiconductor other than wide-bandgapsemiconductors.
 2. The converter according to claim 1, furthercomprising a semiconductor module including the first to sixth diodesand the first transistor.
 3. The converter according to claim 1, whereinrated current of each of the first and second diodes is greater thanrated current of each of the third to sixth diodes and the firsttransistor.
 4. The converter according to claim 1, wherein thewide-bandgap semiconductor is SiC, and the semiconductor other thanwide-bandgap semiconductors is Si.
 5. A power conversion devicecomprising: the converter of claim 1; and an inverter configured toconvert first to third direct-current voltages applied to the first tothird output terminals, respectively, into alternating-current voltagesin three levels to he output to a fourth output terminal, the invertercomprising a second transistor having first and second electrodesconnected to the first and fourth output terminals, respectively, athird transistor having first and second electrodes connected to thefourth and second output terminals, respectively, seventh and eighthdiodes connected in anti-parallel with the second and third transistors,and a second bidirectional switch connected between the third outputterminal, and the fourth output terminal, the second bidirectionalswitch including fourth and fifth transistors and ninth and teethdiodes, the second transistor; the third transistor, the ninth diode,and the tenth diode being each formed of the wide-bandgap semiconductor,and the fourth transistor, the fifth transistor, the seventh diode, andthe eighth diode are each formed of the semiconductor other thanwide-bandgap semiconductors.
 6. The power conversion device according toclaim 5, wherein the fourth transistor has a first electrode connectedto a first electrode of the fifth transistor and a second electrodeconnected to the third output terminal, and the fifth transistor has thefirst electrode connected to the first electrode of the fourthtransistor and a second electrode connected to the fourth outputterminal, the ninth and tenth diodes are connected in anti-parallel withthe fourth and fifth transistors, respectively, when the first and thirddirect-current voltages are alternately output to the fourth outputterminal, the fifth transistor is turned on and the second and fourthtransistors are alternately turned on, and when the second and thirddirect-current voltages are alternately output to the fourth outputterminal, the fourth transistor is turned on and the third and fifthtransistors are alternately turned on.
 7. The power conversion deviceaccording to claim 5, wherein the fourth transistor has a firstelectrode connected to the fourth output terminal and a second electrodeconnected to a second electrode of the fifth transistor, and the fifthtransistor has a first electrode connected to the third output terminaland the second electrode connected to the second electrode of the fourthtransistor, the ninth and tenth diodes are connected in anti-parallelwith the fourth and fifth transistors, respectively, when the first andthird direct-current voltages are alternately output to the fourthoutput terminal, the fifth transistor is turned on and the second andfourth transistors are alternately turned on, and when the second andthird direct-current voltages are alternately output to the fourthoutput terminal, the fourth transistor is turned on and the third andfifth transistors are alternately turned on.
 8. The power conversiondevice according to claim 3, wherein the fourth transistor has a secondelectrode connected to the third output terminal, and the fifthtransistor has a second electrode connected to the fourth outputterminal, the ninth diode has an anode connected to the third outputterminal and a cathode connected to a first electrode of the fifthtransistor, and the tenth diode has an anode connected to the fourthoutput terminal and a cathode connected to a first electrode of thefourth transistor, when the first and third direct-current voltages arealternately output to the fourth output terminal, the fifth transistorIs turned on and the second and fourth transistors are alternatelyturned on, and when the second and third direct-current voltages arealternately output to the fourth output terminal, the fourth transistoris turned on and the third and fifth transistors are alternately turnedon.
 9. The power conversion device according to claim 5, wherein theinverter comprises a semiconductor module including the second to fifthtransistors and the seventh to tenth diodes.
 10. The power conversiondevice according to claim 5, wherein rated current of each of the secondand third transistors is larger than rated current of each of the fourthand fifth transistors and the seventh to tenth diodes.
 11. The powerconversion device according to claim 5, wherein the wide-bandgapsemiconductor is SiC, and the semiconductor other than wide-bandgapsemiconductors is Si.